Nuclear reactor fuel element



April 25, 1961 w. E. KINGSTON 2,981,672 NUCLEAR REACTOR FUEL ELEMENTFiled Jan. 29, 1952 RIIIIM Y INVENTOR 5 WALTER E. KINGSTON I Y B li m/United States Patent Ofitice 2,981,672 Patented Apr. 25, 1961 12,981,672 NUCLEAR REACTOR FUEL ELEMENT Walter E. Kingston, Bayside,N.Y., assignor, by mesne assignments, to the United States of America asrepresented by the United States Atomic Energy Commissron Filed Jan. 29,1952, Ser. No. 268,801 4 Claims. (Cl. 204-1932) This invention relatesto wire fuel elements and their method of manufacture.

Many types and designs of fuel elements have been devised for use withthe various types of nuclear reactors, the most common type being theplate type in which a slab of uranium is sandwiched between two sheetsof zirconium. These fuel elements of the prior art although givingfairly satisfactory results do not provide for opti mum core or surfacearea ratios and do not possess the desired dimensional stability. Adescription of the theory and practice of the design construction andoperation of reactors in which the fuel element of the present inventionmay be used is generally set forth in the Science and Engineering ofNuclear Power, Addison Wesley Press, Inc., Cambridge, Massachusetts,volume I (1947), volume H (1949). Reference is made particularly tochapters 4, 5, 6, 8 and 9 of volume I of that publication.

It is an object of this invention to provide 'a fuel element which willallow for the provision of optimum core-to-surface area ratios.

It is a further object of this invention to provide a fuel element inwhich dimensional instability can be minimized.

It has been found that these objects and other advantages incidental toits application can be attained by making the fuel element in wire form.In this form the core containing fissionable matter is entirely sheathedwithin a metal which preferably has high corrosion resistance and a lownuclear absorption with the metal sheath intimately bonded to the core.

The wire fuel element of this invention can be made in any number ofways. A technique which is well adapted for this purpose includes thesteps of swaging and/or wire drawing. In accordance therewith a billetor slug of material containing fissionable matter is first prepared inaccordance with techniques well known in the art of preparing materialscontaining fission-able material. This billet is then enclosed orsheathed in a material which will withstand corrosion at hightemperatures and which has a relatively low nuclear absorption. Thesheathing is done in such manner that the billet is entirely enclosedwith no parts exposed to the atmosphere. This component is then swagedand drawn at suitable temperature conditions until a composite wire ofthe desired dimensions is obtained.

It should be clearly understood that the wire type fuel element of thisinvention is not necessarily limited to any one specific type ofmaterial containing fissionable matter or to any specific type ofsheathing material. The materials which will be hereinafter specifiedare only given as examples of satisfactory material suitable for use ina specific type reactor.

In a certain type reactor it has, for example, been found to bedesirable to make the core material out of a zirconium-uranium alloy andto clad this with zirconium because of its good corrosion resistingproperties in the presence of Water at temperatures in the neighborhoodof 300 C. Reactors of other types naturally make use of different corematerials and different cladding which may either require more rigid orless rigid specifications and therefore permit the use of othermaterials such as stainless steel, aluminum or beryllium.

In those cases in which the zirconium-uranium alloy is to be used forthe core material in the preparation of the wire type fuel element it ispreferably made by powder metallurgical techniques wherein the requiredpercent of uranium and zirconium are pressed and sintered to optimumdensity and then swaged and machined to a cylindrical shape of therequired dimensions. Such bar could, of course, be rolled into acylindrical form and could also by the use of the proper techniques bemade by casting and rolling.

The method of preparing the zirconium or other sheathing material is notof any real significance provided no contaminants are introduced as adirect consequence of the particular method chosen. It is merelyessential that the resulting product be tubular in form with an insidedimension slightly larger than the outside diameter, of the bar which isto be used as the core material. The tube should preferably be slightlylonger than the bar so that some means can be provided for closing offthe ends with zirconium plugs or by crimping after the bar has beeninserted and therby completely shield it from the atmosphere.

In some cases it is desirable to place an intermediate substance inbetween the core material and the sheath in order to promote goodbonding of the two substances during the formation of the wire. A thinlayer of aluminum is an example of a substance which could be so used.

The wires which are produced in this manner could, for example, be woundin coils and thereby allow considerable rigidity with some freedom ofmovement.

Another arrangement might be one in which the wire is strung in agrid-like arrangement either separated from each other or in tightlypacked arrangement wherein the water is allowed to circulate through thespaces. Figure 1, Figure 2 and Figure 3 of the drawings showarrangements of this type.

Figure 1 is a plan view of the assembly of Figure 2.

Figure 2 of the drawing is a front elevation partly in section showing aspider web assembly in which the wire is used in a coil shape.

Figure 3 is a front elevational of an assembly showing the wire strungin a grid-like arrangement.

In the modification shown in Figure l the tube 10 is made of stainlesssteel upon which 6 lugs have been welded at one end with a similarnumber of lugs at the other end. These lugs 12 are each provided with aslot 14 into which a series of flat stainless steel plates can beplaced. Each of the plates as shown in the drawings at 16 are providedwith staggered semi-circular openings 18 so positioned that when aplurality of plates are laid against one another the semi-circularopenings meet to form a complete circular opening as at 24 the wire fuelelements 20 are wound onto the supporting strips as can readily be seenin the drawings.

In some cases it has been found advantageous to enclose the core andsheath in a tubular shielding member during the swaging and wire drawingsteps. In these cases it is, of course, necessary to remove the outercovering prior to the use of the composite wire as a fuel element.

In general it may be said that wire fuel elements having a goodcore-to-sheath bond and good corrosion resistance can be prepared in anydesired length by proceeding in accordance with the following generaloperations:

(l) Assembling the components of the wire fuel element.

(2) Swaging, stripping and cleaning the assembled element.

(3) Heat treating.

(4) Centerless grinding to give a smooth exterior as well as to cleanand remove contaminating material.

(5) Welding.

(6) Finishing.

In accordance with a preferred method of this invention the wire fuelelement can be assembled, for example, from the following components: Asteel shield having dimensions of 0.625" O.D. x 0.510" I.D., a zirconiumsheath having dimensions of 0.500 O.D. x 0.267" LD. and a zirconiumuranium core having dimensions of 0.257" D. All the components should bethoroughly cleaned before assembly. Acetone has been found to besatisfactory for this purpose. The inside of the steel shield and theoutside of the zirconium sheath should preferably be covered with anoxide which is stable at about 900 C. as, for example, magnesium oxide,zirconium oxide, or beryllium oxide. The material helps to keep down thecontamination of zirconium by iron. The zirconium uranium core is theninserted within the zirconium sheath which is in turn inserted withinthe steel sheath after which the steel tube can be sealed with a steelplug which can be welded to the tube by an argon arc welding process.

This assembly may then be heated in a hydrogen atmosphere or an inertgas atmosphere, up to about 900 C. at which temperature the assembly canbe swaged from 0.625" O.D. to 0.230" O.D. in a number of passes. Theswaging temperature may then be lowered to approximately 800 and swagingcontinued down to a desired size which may be in the neighborhood of0.130. The initial swaging temperature is quite critical if good resultsare to be obtained. It has been found that at temperatures substantiallyabove 900 C. the zirconium sheath picks up suificient iron from theshielding member that the end product is contaminated to a considerabledegree whereas at temperatures below 830 the core material is still toohard to permit the formation of a good sound bond between the core andthe zirconium sheath.

After the wire has been reduced to the proper size the steel may bestripped therefrom (in a 50% nitric acid bath) to completely remove anyiron which may contaminate a portion of the surface. The wire canfurther be pickled in a 5% hydrofluoric acid bath if further cleansingis deemed desirable. After passing through this stripping and cleaningtreatment it may be swabbed with a solution of hydrochloric acid andpotassium ferrocyanide to test for the presence of iron contamination. Ablue color on the swab would indicate the presence of non.

Since the swaged wire frequently shows a string of inclusions along thesheath core interface it is preferable to heat the wire for one hour atabout 900 C. This heat treatment can be carried out either in vacuum orin helium gas. When the material is heat treated in a vacuum tubefurnace the surfaces may again become con taminated in which case thecontaminated layer can be removed with dilute hydrofluoric acid.

When the wire is heat treated in a helium atmosphere it acquires a darkcoating which resists attack by 5% hydrofluoric acid. However, the heattreated wire is attacked by water at 600 F. and it is possible to removethe contaminated layer by liquid honing after which the wire iscorrosion resistant.

Since the wire formed normally has a contaminated surface afterstripping it is often preferable to grind it to give it a smoothexterior. It has been found that when the diameter is reduced about 0.01" by grinding it is usually sufficient to clean up all but the deepestcorrugations and in addition remove most of the contaminated metal.

Wires formed in this manner can be linked to one another by a weldingprocess to form wires of infinite length. To accomplish this it isusually desirable to first protect the ends of the wire by welding a pinof Zirconiurn to each end. The welding method which has been mostsatisfactory to date is the normal resistance welding although theheliarc process has also given good results. When the fuel wires arejoined by resistance welding it has been found preferable to carry outthe welding operation before centerless grinding the wire since the heataffected metal frequently acquires a surface that is susceptible tocorrosion even though the joint may be flooded with helium during thewelding operation. Although this skin is difficult to remove by picklingin dilute hydrofluoric acid it can later be removed during the grindingoperation. Joints which are formed in this way are quite good and themetal can be bent several times through arcs of 30 and 60 without anyindication of weld-line failures. Although welds can be made by theheliarc welding process results obtained up to now have not been assatisfactory as with the resistance welding operation.

In those cases in which the wire is centerless ground to size beforewelding the skin formed during the welding process at the point ofjuncture can be removed by a liquid honing (vapor blasting) process.

While the above description and drawings submitted herewith disclose apreferred and practical embodiment of the method of making the wire typefuel element of this invention it will be understood by the specificdetails of construction and arrangement of parts as shown and describedare by way of illustration and are not to be construed as limiting thescope of the invention.

What is claimed is:

1. In a nuclear reactor a wire fuel element comprising a core of uraniumconcentrically disposed within and bonded to an intermediate sheath ofaluminum and an outer metal sheath bonded to and concentric with saidintermediate sheath, the metal of said outer sheath being selected fromthe group consisting of beryllium, zirconium and stainless steel.

2. The wire fuel element of claim 1 in which the outer sheath isberyllium.

3. The wire fuel element of claim 1 in which the outer sheath iszirconium.

4. The wire fuel element of claim 1 in which the outer sheath isstainless steel.

References Cited in the file of this patent UNITED STATES PATENTS1,465,553 Kirk Aug. 21, 1923 2,063,470 Staples Dec. 8, 1936 2,079,710Jacobson May 11, 1937 2,124,657 Ritzenthaler July 26, 1938 2,253,035Kallmann Aug. 19, 1941 2,386,119 Jack Oct. 2, 1945 2,555,372 Ramage June5, 1951 2,708,656 Fermi et a1 May 17, 1955 FOREIGN PATENTS 648,293 GreatBritain Jan. 3, 1951 OTHER REFERENCES Chipman: Metallurgy in theDevelopment of Atomic Power, U.S.A.E.C. Document No. MDDC-539 (MIT),declassified November 25, 1946. 20 pages. Copy in Scientific Library.

Nature, vol. 166, N0. 4222, September 30, 1950, page 5 35. Copy inScientific Library.

1. IN A NUCLEAR REACTOR A WIRE FUEL ELEMENT COMPRISING A CORE OF URANIUMCONCENTRICALLY DISPOSED WITHIN AND BONDED TO AN INTERMEDIATE SHEATH OFALUMINUM AND AN OUTER METAL SHEATH BONDED TO AND CONCENTRIC WITH SAIDINTERMEDIATE SHEATH, THE METAL OF SAID OUTER SHEATH BEING SELECTED FROMTHE GROUP CONSISTING OF BERYLLIUM, ZIRCONIUM AND STAINLESS STEEL.